Eclipse of Thales

The Eclipse of Thales was a solar eclipse that was, according to The Histories of Herodotus, accurately predicted by the Greek philosopher Thales of Miletus. If Herodotus's account is accurate, this eclipse is the earliest recorded as being known in advance of its occurrence. Many historians believe that the predicted eclipse was the solar eclipse of 28 May 585 BC.[1][2] How exactly Thales predicted the eclipse remains uncertain; some scholars assert the eclipse was never predicted at all.[3][4][5] Others have argued for different dates,[6] but only the eclipse of 28 May 585 BC matches the conditions of visibility necessary to explain the historical event.[7]

According to Herodotus, the appearance of the eclipse was interpreted as an omen, and interrupted a battle in a long-standing war between the Medes and the Lydians. The fighting immediately stopped, and they agreed to a truce. Because astronomers can calculate the dates of historical eclipses, Isaac Asimov described this battle as the earliest historical event whose date is known with precision to the day and described the prediction as "the birth of science".[8]

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Herodotus' The Histories 1.73–74 states that a war started in the period between the Medes and the Lydians. There were two reasons for the war: the two sides had clashing interests in Anatolia, but also there was a motive of vengeance: some Scythian hunters employed by the Medes who once returned empty-handed were insulted by King Cyaxares. In revenge the hunters slaughtered one of his sons and served him to the Medes. The hunters then fled to Sardis, the capital of the Lydians. When Cyaxares asked for the Scythians to be returned to him, Alyattes refused to hand them over; in response, the Medes invaded.

Afterwards, on the refusal of Alyattes to give up his suppliants when Cyaxares sent to demand them of him, war broke out between the Lydians and the Medes, and continued for five years, with various success. In the course of it the Medes gained many victories over the Lydians, and the Lydians also gained many victories over the Medes. Among their other battles there was one night engagement. As, however, the balance had not inclined in favour of either nation, another combat took place in the sixth year, in the course of which, just as the battle was growing warm, day was on a sudden changed into night. This event had been foretold by Thales, the Milesian, who forewarned the Ionians of it, fixing for it the very year in which it actually took place. The Medes and Lydians, when they observed the change, ceased fighting, and were alike anxious to have terms of peace agreed on.[9]

As part of the terms of the peace agreement, Alyattes's daughter Aryenis was married to Cyaxares's son Astyages, and the Halys River (now known as the Kızılırmak River) was declared to be the border of the two warring nations.

An alternative theory regarding the date of the battle suggests that Herodotus was carelessly recounting events that he did not personally witness, and that furthermore the solar eclipse story is a misinterpretation of his text.[10] According to this view, what happened could have been a lunar eclipse right before moonrise, at dusk. If the warriors had planned their battle activities expecting a full moon as in the previous few days, it would have been quite a shock to have dusk fall suddenly as an occluded moon rose. If this theory is correct, the battle's date would be not 585 BC (date given by Pliny based on date of solar eclipse), but possibly 3 September 609 BC or 4 July 587 BC, dates when such dusk-time lunar eclipses did occur.[10]

At the time of Thales' purported prediction it was not yet known that eclipses were caused by the Moon coming between the Earth and the Sun, a fact that would not be discovered until over a century later by either Anaxagoras or Empedocles.[11]

If the account is true, it has been suggested that Thales would have had to calculate the timing of any eclipse by recognizing patterns in the periodicities of eclipses.[6]
It has been postulated that Thales may have used the Saros cycle in his determination, or that he may have had some knowledge of Babylonian astronomy. However, Babylonians were far from being able to predict the local conditions of solar eclipses at that point, which makes this hypothesis highly unlikely.[4] In fact, there is no known cycle that can be reliably used to predict an eclipse for a given location and, therefore, any accurate prediction would have been down to luck.[6][5]